mem: Track the MemObject owner in MasterPort and SlavePort.

mem: Track the MemObject owner in MasterPort and SlavePort.

These types are much more tied to MemObjects and the gem5 memory
protocol than the Port or BaseMasterPort and BaseSlavePort classes.

Change-Id: I36bc8c75b9c74d28ee8b65dbcbf742cd41135742
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17032
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>